The present invention relates generally to encapsulants for opto-electronic devices, and more particularly, to such encapsulants exhibiting superior optical qualities. The present invention also relates to methods for making such encapsulants.
Optical and electronic (i.e., “opto-electronic”) devices, such as LEDs, photodetectors, and fiber optic components, generally are encapsulated using a variety of materials to protect the devices from vibration, humidity, heat, environmental deterioration, electrical leakages, and other deteriorative factors. A suitable encapsulant should possess a number of particular characteristics, described in detail below.
The encapsulant should have a coefficient of thermal expansion (CTE) preferably lower than 50 ppm/° C. Also, variation in the CTE should be lower than ±30% over the entire volume of the encapsulant. If the CTE and/or its variation are greater than these values, the encapsulant can excessively expand or contract when exposed to varying temperatures, thereby causing breakage of the device or its leads.
The encapsulant also should have the highest possible light transmittance, preferably at wavelengths between 300 nm and 800 nm. In particular, the encapsulant preferably should exhibit light transmittance higher than 65% at about 650 nm for an encapsulant having thickness of about 1 mm. In contrast, commercially available encapsulants often have substantially lower transmittance, and therefore are translucent, or even opaque. This can lead to difficulty when these encapsulants are used with opto-electronic devices requiring light transmission.
Additionally, the encapsulant should have a glass transition temperature (Tg) preferably higher than 120° C. Because opto-electronic devices are subjected to temperatures substantially higher than usual ambient temperatures, the encapsulant can flow when its glass transition temperature is exceeded, resulting in destruction of the encapsulated device.
The encapsulant also should have high electrical resistivity, to provide sufficient insulation for the opto-electronic devices. It should further provide adequate electrical insulation to protect the opto-electronic devices from the effects of adverse environmental conditions, such as heat and humidity. Finally, the encapsulant should be inexpensive and easy to produce using readily available materials.
Transparent epoxy resin compositions having refractive indices (i.e. nD at 25° C.) varying between 1.48 and 1.60 at a wavelength of about 588 nm can be prepared by curing commercially available chemical compounds incorporating epoxy groups. These inexpensive commercial epoxy resins also are known to exhibit high light transmittance, high Tg, high electrical resistivity, and high heat resistance. However, these epoxy resins usually have CTE higher than 50 ppm/° C., and therefore they are not suitable for use as encapsulants for opto-electronic devices.
Encapsulants having low CTE can be prepared by mixing commercially available uncured epoxy compounds with inorganic fillers, followed by curing of this mixture. For example, U.S. Pat. No. 3,547,871 to Hofman et al. describes a low CTE encapsulant comprising an epoxy resin and a filler having a particle size ranging between 10 μm and 300 μm. This encapsulant resin has a viscosity below 20,000 cP at 100° C. The filler is selected from silica, fused quartz, beryllium aluminum silicate, lithium aluminum silicate, or mixtures of these. The claimed encapsulants have a CTE lower than 50 ppm/° C. However, the Hofman '871 patent does not disclose an encapsulant having high light transmittance.
An encapsulant having both low CTE and high light transmittance can be prepared by incorporating a filler and epoxy having the characteristics described as follows. The filler should be uniformly dispersed in the epoxy to provide a uniform filler CTE over the entire volume of the encapsulant, to prevent eventual damage of the opto-electronic device from the cumulative effect of operation at varying temperatures. To obtain uniform particle dispersion and produce an encapsulant having a uniform CTE, the filler should have a predetermined and consistent particle size, and the liquid epoxy should have a predetermined viscosity. If the filler particle size is smaller than about 1 μm, the particles tend to agglomerate, resulting in non-uniform particle dispersions and entrapment of gas bubbles in the encapsulant. This results in lowered light transmittance of the encapsulant. Such agglomeration and bubble formation might be avoided by reducing the amount of the filler used in the encapsulant. However, reducing filler amount tends to increase the CTE of the encapsulant above 50 ppm/° C. Therefore, preferably agglomeration is avoided by using fillers having particle sizes greater than about 1 μm. Unfortunately, if filler particle size is larger than about 1 μm, the particles can settle at the bottom of the encapsulant during preparation and casting of the encapsulant due to gravity. This settling will occur if epoxy viscosity is too low during preparation of the encapsulant layer, leading to non-uniform distribution and non-uniform CTE in the encapsulant. Conversely, if epoxy viscosity is too high, forming the encapsulant layer over the opto-electronic device can be difficult. Therefore, the viscosity of the liquid epoxy should be within a specified range in order for the material to be useful in preparation of the encapsulant.
In addition to uniform dispersion, the refractive index of the filler should closely match the refractive index of the epoxy resin at the cured stage to have high light transmittance. Inorganic fillers frequently are used to lower costs or enhance mechanical properties of the resins. However, their use also can lead to decreased optical transmittance and scattering of light by the medium produced. This scattering can be decreased, and the optical transmittance of the medium can be increased, by closely matching the refractive index of a transparent filler used with that of a transparent resin. Because the refractive index of these filler particles generally cannot be directly measured with sufficient precision, this matching generally must be performed by trial and error, adjusting the filler or epoxy as needed.
Also, the filler should be free of chemical compounds that can reduce the electrical resistivity of the encapsulant below an acceptable level under the temperature and humidity conditions of the device. Some of these compounds have inherently low electrical resistivity, while others can decompose into or form electrically conductive ions. Heat and humidity can affect this inherent electrical resistivity or this decomposition, as well as migration of the ions. Therefore, preparation of the filler from such compounds should be avoided. Finally, the filler should be manufactured easily and inexpensively.
Single-component particles of inorganic metal oxides, such as SiO2, TiO2, and ZrO2, are known to be easily and inexpensively prepared using vapor deposition or solution precipitation processes. These particles can be used as fillers in encapsulants. However, these particles are often less than 1 μm in size, and therefore generally are unsuitable for use as fillers for the reasons described above. Furthermore, most single-component particles cannot be used as fillers, because they have fixed refractive indices. For example, SiO2 particles have a fixed refractive index of 1.42, TiO2 (rutile form) of 2.3, and ZrO2 of 1.95. These indices cannot be adjusted to match the refractive index of the epoxy resin used for making the encapsulant.
It is possible to adjust the refractive index of fillers made from multi-component glass particles. For example, U.S. Pat. No. 5,175,199 to Asano et al. describes a sol-gel method for making a multi-component glass filler to be mixed into a transparent epoxy, which can be used as an encapsulant for optical semiconductor devices. In this method, TiO2—SiO2 gel is synthesized by hydrolyzing and condensing a silicon alkoxide and a titanium alkoxide. This gel is dried, and then either ground into particulate matter, followed by sintering to dense glass beads, or sintered into a dense glass followed by grinding into glass beads. This sintering is achieved at a very high temperature range of 1,050° C. to 1,250° C. The disclosed filler manufacturing method has the disadvantages of being complicated and expensive, as well as requiring lengthy preparation time and high sintering temperature. Therefore, production of these fillers is expensive. Additional disadvantages include possible phase separation, crystallization, and coloring of TiO2 at the high sintering temperatures required using these methods. Phase separation and crystallization cause intolerably high refractive index differences between the glass filler and the epoxy, thereby lowering the transmittance. TiO2 is also known to possibly cause yellowing of organic resins in which it is included as a result of extended exposure to light. This leads to degradation of the transmittance of the encapsulant over time.
U.S. Pat. No. 5,198,479 to Shiobara el al. uses the method described in the Asano '199 patent described above, and it further discloses a method to overcome the discoloration problems of the TiO2—SiO2 fillers discussed above, by addition of organic phosphorus anti-discoloring agents into the uncured epoxy-filler composition. This addition, while effective, further complicates use of the method described in the Asano '199 patent, and the resulting filler therefore is more expensive.
European Patent No. 0 391 447 B1 to Nakahara el al. teaches a sol-gel method for the production of multi-component metal oxide particles that can be used as fillers in transparent organic resins. The Nakahara et al. patent incorporates the step of first preparing seed particles of single-component metal oxides, then growing these particles by addition of hydrolyzable and condensable organic metal compounds such as metal alkoxides, to prepare multi-component particles such as TiO2—SiO2, ZrO2—SiO2, and Al2O3—SiO2. This process is complicated and expensive, and the fillers produced are expensive. Furthermore, because these particles are smaller than 1 μm in size, it is difficult to obtain both homogeneous bubble-free dispersions of these particles in the epoxy and low CTE.
U.S. Pat. No. 5,618,872 to Pohl et al. discloses a method for making multi-component encapsulant filler particles for opto-electronic devices, comprising two or more oxides selected from SiO2, TiO2, ZrO2, Al2O3, V2O5, and Nb2O5. The method for particle preparation described in this patent is similar to that described in the Nakahara et al. patent, and it shares the same drawbacks. Therefore, it is not suitable for preparation of high transmittance encapsulants.
Dunlap and Howe, in Polymer Composites, vol. 12(1), pp. 39-47, (1991), describe a casting composition comprising a resin and an index-matched filler prepared by ball milling of a glass. The size of the filler particles ranges between 2 μm and 100 μm. Subsequent to ball milling, the filler particles are annealed at temperatures between 0° C. and 10° C. above the glass strain point for at least one hour to remove stresses, as well as organic contaminants. The inventors have found that heat treatments at temperatures above the strain point of the glass can reduce transmittance, and therefore such temperatures should be avoided. Furthermore, Dunlap and Howe do not disclose an encapsulant having a uniform CTE and a method for preparing such an encapsulant.
Japanese Patent Publication No. 11-074424 to Yutaka et al. discusses a method for making an encapsulant for use in a photosemiconductor device. In this method, a silica powder containing PbO or TiO2 having a particle size ranging between 3 μm and 60 μm is used as an index-matched filler for an epoxy resin composition. PbO or TiO2 can cause crystallization during manufacturing of these multi-component glass fillers, thereby decreasing the transmittance of the filled epoxy. In addition to the aforementioned disadvantages of using TiO2 as a filler material, PbO is known to be a health hazard, and its use in manufacturing of the encapsulants should therefore be avoided.
U.S. Pat. No. 6,246,123 to Landers et al. describes an encapsulant having high transmittance, low CTE, and low Tg. The encapsulant is made from a polymer resin and an index-matched filler. However, the filler is selected from a group consisting of alkali zinc borosilicate glasses. The presence of alkali ions is know to potentially reduce electrical resistivity of encapsulants, leading to high leakage currents and possible damage to the encapsulated device. For example, U.S. Pat. No. 4,358,552 to Shinohara et al. explains that encapsulants incorporating low levels of alkali contaminants, such as Li+, Na+, K+, and ionic contaminants, such as Cl, improves electrical insulation of the encapsulated electronic device.
Naganuma et al. in Journal of Material Science Letters, vol. 18, pp. 1587-1589, (1999) describes preparation of encapsulants for opto-electronic devices by mixing an epoxy with a filler prepared from a multicomponent glass, SiO2—Al2O3—B2O3—MgO—CaO having an average particle size of 26 μm or 85 μm. During the described curing of the epoxy and the filler mixture, the mold is turned over every 10 minutes to prevent segregation of the filler. However, these encapsulants do not have transmittance higher than 65% at a CTE lower than 50 ppm/° C., and therefore are not suitable for use in the manufacture of opto-electronic devices.
It should be appreciated from the foregoing description that there remains a need for a transparent encapsulant having uniform and low CTE over its entire volume comprising inexpensive index-matched fillers and epoxy resins. The fillers should be able to be uniformly dispersed to provide an average CTE lower than 50 ppm/° C., with a CTE variation of less than ±30% over the entire volume of the encapsulant to prevent thermal expansion damage. The encapsulant also should have a transmittance higher than 65% to minimize signal losses, and Tg higher than 120° C. to reduce physical stress and breakage, thereby preventing the opto-electronic device from damage. Finally, the encapsulant should exhibit high electrical resistivity under varying temperature and humidity conditions. The present invention fulfills this need and provides further advantages.